Unlike primary batteries, secondary batteries have the function of generating electric energy repeatedly after charge. Lead batteries are known as typical examples of the secondary battery. In lead batteries, lead oxide, lead, and sulfuric acid are used as the active substance at the anode, the active substance at the cathode, and the electrolyte, respectively. Lead batteries are widely used in industrial products, such as automobiles, battery fork lift trucks, emergency power sources, and UPS (alternating current unperturbed power supplies), and as batteries in household use, such as small sealed lead batteries. Because of the economic advantage, lead batteries are also used as the power source of electric automobiles which are expected to become popular as vehicles causing no environmental pollution.
On the other hand, as batteries become smaller and lighter and also as portable instruments are widely used, demand for sealed alkali secondary batteries, particularly nickel-cadmium secondary batteries, is increasing as a back-up power source for instruments related to information processing and a power source for portable electric instruments. Recently, nickel-hydrogen secondary batteries and lithium ion secondary batteries are also in the stage of practical application. The sealed alkali secondary batteries are expected to be used increasingly as the battery for electric automobiles.
In the sealed secondary batteries such as those described above, a battery jar which contains an electrolyte of an acid or an alkali and electrodes is indispensable. A battery jar is required to have resistance to electrolytes, i.e., resistance to acids and alkalis; resistance to solvents, i.e., resistance to oil and grease; resistance to heat and pressure which is necessary for remaining stable at increased temperatures caused by heat of chemical reactions at the time of charge and discharge and at increased inner pressures caused by generation of gases; mechanical strengths which are necessary for sufficiently withstanding impact from the outside; electric insulation; and a light weight which is necessary for producing a smaller and lighter battery.
Heretofore, metals have been used as the battery jar material for sealed secondary batteries. However, although battery jars made of a metal have advantages in large mechanical strengths and good heat resistance, pressure resistance, and solvent resistance, these battery jars have drawbacks in that they show inferior chemical resistance and poor electric insulation, require complicated working because welding is necessary, and have heavy weights. Therefore, application of various types of plastics, such as polystyrene, polypropylene, ABS resins, AS resins, poly-4-methylpentene-1, and blends of HIPS and a polyphenylene ether, as the battery jar material has been attempted (the specifications of Japanese Patent Application Laid-Open No. Showa 64(1989)-65771, Japanese Patent Application Laid-Open No. Showa 57(1982)-19965, Japanese Patent Application Laid-Open No. Heisei 6(1994)-231738, and Japanese Patent Application Laid-Open No. Heisei 6(1994)-203814). Although battery jars made of these plastics have advantages in that they have light weights and show easy working and good electric insulation, these battery jars have following drawbacks and are not always satisfactory. For example, battery jars made of polystyrene or an ABS resin show insufficient heat resistance, pressure resistance, and solvent resistance. Battery jars made of polypropylene show insufficient heat resistance, pressure resistance, and solvent resistance at high temperatures. Battery jars made of poly-4-methylpentene-1 show insufficient strengths at high temperatures and solvent resistance. Battery jars made of a blend of HIPS and a polyphenylene ether show insufficient solvent resistance.